An electrical power network is operative at a nominal line or mains frequency. The nominal frequency is 50 Hz in Europe, 60 Hz in North America and 50 Hz or 60 Hz elsewhere. All electric power systems experience oscillations because of the fundamental dynamic nature of generation and load interconnected through a network. Where, for example, the nominal frequency is 60 Hz grid oscillation may cause the frequency to vary from 60 Hz such as between 59 Hz and 61 Hz with a period of 20 seconds; such variation is, however, an extreme example. Grid oscillation may thus be considered a modulation of a measured signal with another signal of different and normally considerably lower frequency than the nominal frequency. Oscillation in measured frequency implies that there are oscillations in generator rotor angles and consequently the angle component of phasors representing the magnitude and angle of voltage at the fundamental grid frequency.
For example the oscillations may cause the voltage angle to oscillate by an amplitude of 1 degree with a period of 2 seconds.
Grid oscillations arise on account of interactions between and amongst grid subsystems with each of the involved subsystems contributing either positively, negatively or neutrally to the grid oscillation. Of grid subsystems, generators are often significantly involved in grid oscillation on account of generator rotor speed variation. Several mechanisms can result in generators contributing to the energy of an oscillation, including the action of Automatic Voltage Regulators (AVR) and turbine governor regulators. Other control systems, such as Power System Stabilisers (PSS), act to improve the stability of the oscillation by providing damping. Grid oscillations are evident not only from oscillations in the frequency but also from oscillations in the like of line power, voltage angle (which is normally referred to as angle), angle difference between geographically separate parts of the grid, and the speed of rotating machines, such as generators, which form part of the grid. Use of the term voltage angle herein means the representation of a voltage waveform at the fundamental frequency of the grid with respect to a time-synchronised reference as defined by IEEE C37.118 (2005) and in particular in Section 4.2. Grid oscillations are thus evident from variations in the AC electrical characteristics of the affected part of the grid and from variations in the mechanical characteristics of rotating machines comprised in the affected part of the grid.
Grid oscillations can, under certain circumstances, be prejudicial to proper operation of an electrical grid in particular where the grid oscillations are poorly damped if not unstable. Grid oscillations may, for example, cause protection relay tripping which leads to loss of interconnecting lines or loss of load, or generator or turbine stresses. FIG. 1 shows an unstable oscillation in grid frequency that led to system separation and load-shed relay tripping. Considering FIG. 1 more closely, a graph of grid frequency over time is shown which reflects a nominal 60 Hz system oscillating at low frequency with a period of 20 seconds so as to cause variation in the frequency between 59 Hz and 61 Hz.
Grid oscillations often involve and affect many generators in an electrical grid. Furthermore grid oscillations may span boundaries between network areas controlled by different operating companies and may perhaps span national boundaries. It is therefore desirable to identify those grid subsystems that are contributing negatively to grid oscillation so that appropriate action can be taken. Appropriate action may, for example, involve control room initiated real-time generation re-dispatch or planned action such as tuning a controller in a negatively contributing subsystem. Furthermore information on relative contributions of various subsystems to grid oscillation may prove useful in addressing inter-company or indeed international jurisdictional issues arising from the grid oscillation.
The present inventors have appreciated that it is at present difficult to determine to what extent grid subsystems, such as generators or control apparatus, are contributing to grid oscillation, such as by contributing positively or negatively or merely responding to the grid oscillation.
It is therefore an object for the present invention to provide a method of determining a contribution of at least one grid subsystem to oscillation in angle in an electrical grid.
It is a further object for the present invention to provide apparatus for determining a contribution of at least one grid subsystem to oscillation in angle in an electrical grid.